1. Field of the Invention
The present invention relates to a lens control apparatus configured to control the position of a zoom lens and a focus lens, to a lens barrel, to an imaging apparatus having the lens barrel and the lens control apparatus, and to an optical apparatus.
2. Description of the Related Art
Conventionally, a lens barrel for a still camera or a video camera has been widely used that drives a zoom lens and a focus lens, each including a lens unit, with a driving member, such as a stepping motor.
Generally, in the case of driving a lens unit with a driving member, such as a stepping motor, an open loop control method is often used as a control method for driving and positioning the lens unit. This is because in the open loop control method, it is not necessary to provide a detection device for detecting the current position of the lens unit. In addition, this is because in the open loop control method, the configuration of a control system can be more simplified and the size of the apparatus can be relatively small compared to a control system of the closed loop control type.
However, in the case of performing positioning control on a lens unit with the open loop control method that uses a stepping motor, it is necessary to perform control so that a drive start position of the stepping motor corresponds to a movement start position of the lens unit. Accordingly, in this case, it is required that each lens unit is provided with a reference position detection unit configured to move a lens unit back to a predetermined reference position before starting the positioning control and to determine whether the lens unit has been positioned back to the reference position (reset position).
Generally, with respect to the shape of a moving locus (a cam locus) of a focus lens, the cam locus goes along a hill-like shaped continuous curve as in the case of a cam locus at an infinite object distance. More specifically, in this case, the cam locus continues to rise from a wide-angle end to a middle position, then comes along a gentle upward curve to reach a peak at the middle position, and goes downward from the middle position to a telephoto end.
As is commonly and publicly known, the cam locus goes along a steeper curve from the middle position to the telephoto end as it approaches the telephoto end, which is characteristic to the cam locus.
Meanwhile, in recent years, in an optical apparatus such as a camera, the size of a lens barrel and the image size of a solid-state image sensor have been more and more decreased. Furthermore, a plastic material is often used as a material of a retaining member for a lens barrel and an optical system.
In this regard, it is useful to use a plastic material as a material of the retaining member for a lens barrel and an optical system because the retaining member can be easily molded with a mold. Furthermore, in this case, the shape of the retaining member can be arbitrarily selected. In addition, the manufacturing costs can be reduced compared to costs for manufacturing a retaining member made of other materials, such as a metal material.
On the other hand, in this case, a problem may arise in that the physical property and size of the retaining member made of a plastic material significantly change due to a high sensitivity to the change of temperature or moisture. In this regard, if a plastic material is used as a material of a component of a lens barrel, the focal length and/or in-focus position more significantly changes than in the case of using a metal material. In this case, the optical performance may degrade due to a phenomenon of defocusing that may arise due to the change in the focal length and in-focus position.
In order to solve the problem like this, Japanese Patent No. 3,581,513 discusses a method for correcting the position of a focus lens according to a result of calculating a defocusing amount based on the temperature change amount.
If the optical magnification is simply increased and the size of an optical system is simply reduced, then the above-described steep slope of the cam locus around the telephoto end may become more significant.
Accordingly, when the optical magnification is high, in an optical system that controls the position of a focus lens along the cam locus, if the position of the zoom lens at the telephoto end is displaced even slightly due to any particular cause, it becomes necessary to move the focus lens by an amount several tens of times as large as the amount of displacement of the position of the zoom lens at the telephoto end.
The most significant cause for such defocusing is a possible thermal expansion of the component member for a lens unit occurring due to the temperature rise. If the zoom lens moves to a position beyond the telephoto end due to the thermal expansion occurring due to the temperature rise, then it becomes necessary to move the focus lens towards an image plane (towards an image sensor, such as a charge-coupled device (CCD) or a complementary metal oxide semiconductor (CMOS)) by a distance several tens of times as great as the distance of the movement of the zoom lens beyond the telephoto end to achieve an in-focus state.
In this case, with respect to the mechanical dimension of a lens barrel, it becomes necessary to provide a clearance greater than the above-described distance of moving the focus lens to achieve an in-focus state. Accordingly, in this case, the entire mechanical length of the lens barrel may become long and the size thereof may become large.